In Warsaw, Missouri, there's a ghost who keeps talking to me through the mouths of strangers. He is the ghost of slavery past, and he casts a long shadow, even across the streets of this cheerful little lakeside town on a sunny autumn day. A local Obama campaign volunteer tells me about a woman she had canvassed who said she personally would vote for Barack but that her daughter wouldn't - and then the mother lowered her voice - "because he's black". Nor would her son: "he's even more racist". How horrible to feel impelled to say that of your own children. The jokey-scary commercial paraphernalia of Halloween is all around, but here are America's real ghosts and witches.

Missouri matters. It is a national weathervane. Located bang in the middle of the American heartland, where east meets west and north meets south, over the past hundred years it has chosen the winner in every presidential race except one. In the opinion polls, it's among the few states that are still too close to call. That's why Obama was here speaking to massive rallies a fortnight ago, and why both he and Joe Biden are back here again this Thursday. That's why the Obama organisation in Missouri plans to use its 25,000 volunteers to knock on some 1.3m doors during the last four days of the campaign.

Most of those key swing voters are in the sprawling, laundered suburbs of St Louis and Kansas City, but every vote from these rural areas, whose native sons include one of the greatest Democratic presidents, Harry Truman, will count too. And I'm in the heart of the rural heartland: beautiful, gently rolling country, with dawn mist rising from cattle ponds, trees turning every impressionist's shade of autumn russet, yellow and red, cows picturesquely munching lush grass, and roadside signs proclaiming Dirt For Sale, and Jesus Is Lord.

On the corner of Van Buren and Kosciusko street (Tadeusz Kosciuszko, that is, the Polish freedom fighter who inspired the town's name), I notice a neat, white-painted house with a sign in the window saying "This House Protected by God". Out front, a guard dog barks. (A dog called God?) And there's another sign on the lawn: For Sale. The Lord may provide, but people have housing and money worries here as everywhere. And they don't just hunt for the sport. A good shot can put a nourishing turkey or quail on the table for dinner. So the Republicans claim Obama wants to take away your gun. A McCain advertisement on the local country music radio station declares, in a deep countryman's voice, "We love our God and we love our guns" - and you can almost hear a second capital G. And, it goes on, "liberals" want to take them away, being "out of touch with our America".

I had expected race to be an issue here, but I'm struck by how close to the surface the old wounds and prejudices are. I don't even have to ask; it just keeps coming up. At the local headquarters of the McCain campaign, four warmly hospitable local ladies tell me about their enthusiasm for Sarah Palin. When the talk turns to the inevitable subject, one of them says people are afraid of being thought racist if they come out against Obama.

Another recalls how in her childhood, not so far from here, the Ku Klux Klan was still active, and there were roads a black man could not safely walk.

They add that 19th-century Warsaw was a slave town, but Cole Camp, founded by German Lutherans just a few miles to the north in the same county, was not. So Missourians fought about it during the civil war, in the course of which Warsaw was several times burned and razed to the ground.

Up the road in Sedalia a former army officer, for many years a staunch Republican, tells me he will vote for Obama. He's disgusted at the way the Bush administration lied to them about Iraq. But it would be easier if Obama were white. In fact, he would find it difficult to vote for him if he were really African-American "That's black slave American", he helpfully explains to this foreigner. Those people are so "mad" inside, he says, using the word in the colloquial American sense. Fortunately, Obama's not really an African-American, just an American with an African father. But still, he feels "queasy" about that.

Now don't get me wrong. I'm emphatically not here as a condescending urban liberal, a cultural tourist from the Guardian, hellbent on sneering at these sad, backwoods rednecks, and maligning them as racists. Far from it. These were decent, honest, warm-hearted people I met, and they were acknowledging and frankly wrestling with the problem of residual racism, not propagating it. Nor am I leaping to any conclusion so simplistic as "race will decide this presidential race". Mine was a wholly unscientific sample of about 1% of the population (which is 2,070, according to the road sign) of one small town in the conservative rural area of one swing state.

Two impressions, however, I do want to share with you. First, for all the consensus of perhaps the most sophisticated pollsters and pundits in the world that Obama is firming up a solid lead in the electoral college, it seems to me that there are unknowns unique to this election, known unknowns and unknown unknowns, which could still swing it either way. If there are too many secret doubts in too many hearts about the otherness of Obama, McCain could just scrape in. If the superbly organised Obama ground campaign gets out voters whom the pollsters may never have reached - young, poor, ethnic minority, even homeless (a judge in Ohio has just allowed homeless voters to register a park bench as their address) - it could be an amazing landslide. I know only enough to doubt the wisdom of any who say they know.

You did not read it here first. Next week we will all be wise after the event.

The other thing about this election is that, because of the extraordinary Obama and his extraordinary neighbour-to-neighbour campaign, it has become a vast national conversation, not only about the US's future but also about its difficult past. The map of Missouri is weirdly strewn with old European names: Warsaw, Dresden, Windsor, Odessa, Versailles (correct pronunciation: Ver-sails). Old European cities with a lot of history, including much bloodshed and ethnic conflict. But I doubt that in any of them today, perhaps not even in Warsaw, Poland, the wounds of old wrongs still go as deep or throb as hard as they do in their quiet Missourian homonyms, where nice middle-aged Republican ladies can tell you at once who did what to whom nearly 150 years ago.

The Obama campaign may prefer to concentrate on the future, but this difficult conversation about America's past is itself also about its future.

It's painful and may even be a little risky, but it brings the possibility of healing, especially if enough Americans overcome their secret doubts, their "queasiness", and follow Obama's intriguingly worded appeal to "come together as one nation, one people, and once more choose our better history".